Epstein-Barr virus (EBV) infects human B cells and induces their immortalization. The molecular basis for EBV immortalization is largely unknown. Recently, it was observed that when EBV immortalized cells are cultured without serum at critically low densities, proliferation ceases and the cells die. However, addition of cell-free supernatants from-exponentially growing EBV-immortalized cell lines can rescue these cells from death and promote their proliferation. Recently, we have identified IL-6 as being one of the autocrine growth factors for EBV-immortalized B cells (Tosato, G. et al, J. Virol., 1990) However, only approximately 25% of autocrine growth factor activity is attributable to IL-6 while the remaining autocrine growth factor activity resides in a low molecular weight component. The goal of the present study is to identify the molecule or molecules responsible for this low molecular weight autocrine growth factor. So far, we have gathered a great deal of information but have not as yet identified the structure of this low molecular weight growth@factor activity. We have determined that its relative molecular weight is greater than 200 and smaller than 500. We have also determined that it is not a peptide because treatments designed to destroy the peptide bond fail to reduce its biological activity. Purification of the low molecular weight autocrine growth factor is underway and a number of steps have been identified, including size fractionation, anion exchange chromatography and counter current electrophores. Future efforts will concentrate on further purifying the molecule from contaminants and determining its structure.